spectral regrowth after passing QPSK through RF amplifier

I know that RF amplifiers are non-linear amplifiers. I just couldn't understand the concept of spectral regrowth.
Assume that we have a QPSK signal and there are many zero crossings. What is the reason for getting higher frequency band (spectral regrowth) after passing it through the non-linear amplifiers? How does having lesser amount of zero crossings (or in other words having constant envelope) avoid this spectral regrowth after it is passed through the non-linear amplifier?

Someone please reply.

Thanks in advance.